Many amplifiers or comparators are tied to busses which carry data for circuitry other than that supplied by a particular amplifier. In such a case, and in other cases, it is desirable for the amplifier to provide a high impedance output, commonly known as tri-state. One approach is simply to place a switch between the bus and the amplifier. This works well for the high impedance mode of operation but provides undesirable series resistance between the output of the amplifier and the bus during normal operation of the amplifier. During high impedance operation it is also desirable for the amplifier to not have any current flow. This result can be provided by switching off a transistor in series with a power supply. Although this will effectively disable the amplifier, it does not provide the necessary high impedance output so other means have still been required for this purpose. One reason for this is that in the case of double-ended output amplifiers, the two outputs must be isolated from each other which is not achieved by switching a transistor in series with a power supply.